Electrical equipments such as arc resistant switchgears are usually designed to withstand the pressures and temperatures of gases associated with an internal arcing fault. For this purpose, they are equipped with exhaust chambers or plenums which channel exhaust products such as hot gases, debris and metal parts away from the room where the arcing electrical equipment and handling personnel are present.
Usually a plenum assembly provided on top of the switchgear so that the hot gases flow out through the plenum and to outside of the building through the opening provided in the wall. Many-a-times, there exists a clearance distance between the opening of the wall and the plenum assembly termination and the clearance distance varies at various different locations depending on the distance of the wall form the switchgear. Considering that commonly available plenums are of standard lengths, such clearances could result in a gap creating an escape route for hot gases within the building itself which can lead to serious consequences during an arc flash. At present, the only available method to avoid such clearances is to make plenum of customized length as per requirement.
The occurrence of arcing inside electrical switchgear has many undesirable results. The arc energy can cause a sudden pressure increase inside the enclosure resulting in severe mechanical and thermal stress on the equipment and its panel parts. As arcing begins, gases inside the switchgear ionize. The ionization reduces the ability of air inside the switchgear to provide its previous insulative capabilities, and the ions generated by the degradation of the air molecules inside the switchgear cause the air to more readily conduct electricity leading to arc flashes. An arc flash is a phenomenon in which large amounts of light and waves of heat energy are released explosively from electrical equipment. The electric arcs produced by these sources jump anywhere from a few inches to several feet, depending on the voltage level involved and can result in an explosion with a shockwave. If an individual is nearby the unit where are flash occurs, they can be seriously injured, if not killed. Since the human body is a great conductor of electricity, the are travels through human body literally seeking ground resulting in severe injuries.
In case of switchgears having more than one vertical section, there is a possibility of occurrence of secondary fault. An electrical arc contains conducting ions. When this arc travels outside the electrical equipment it can pass through a healthy busbar section usually to the adjacent vertical section. This can cause another fault known as secondary fault which may be more severe than the first arc fault. For example as seen in FIG. 1, which depicts a switchgear 10 comprising vertical sections 11, 12 and 13 and having a plenum 15, in which a fault 14 occurs in section 12. The arc travels out of section 12 through the roof 17 and enters into section 11 to create secondary fault 16. Similarly, the secondary fault can occur in more sections.
Presently available prior art solutions provide arc resistant cabinets which typically include a frame having a hinged flap assembly which open to relieve the pressure inside the cabinet when an arc occurs. In U.S. Pat. No. 6,407,331, as depicted herein in FIG. 2A a pressure release panel 36 is provided for use with arc resistant cabinets. The pressure relief panel 36 includes at least one flap 38 defined by a plurality of cuts 40 within the panel. The patent further describes that during normal operation of the circuit breaker, the flaps remain closed (FIG. 2A). In the event of arcing, as seen in FIG. 2B the flaps open to approximately vertical position by the high pressure expanding gases.
Another prior art, WO2012/088009 describes a vent flap member which opens vertically to permit gases into the opening into the arc chamber. Referring to FIG. 2C depicting the binge assembly of this prior art where each flap 42 is hinged at the junction of the housing and a bottom edge of the flap 42 via two overlapping members. The flaps 42 define at least one detent opening 47 and the flaps 42 are affixed to a surface, of the housing, which has no detent opening. The detent openings of the flaps 42, received the knuckle of the hinge 53 that interlocks with at least one pin structure 51 to form the hinge structure 46. This hinge structure 46 allows the flaps 42 to lay flat generally sealing against the housing surface in both directions of rotation about this hinge structure 52 when in closed position thereof.
Therefore, in actuality, the flap door is typically forced open due to the force of high pressure expanding gases generated during arcing. This delay in release of exhaust material caused due to delay in opening of flap door can lead to the metal housing of the cubical to burn through. While it is apparent that the problem associated with arcing in electrical switchgear has been recognized, it is also readily apparent that the need continues to exist for arc resistant switchgear. Attempts have focused on lessening the chance of an arc occurring, but when it occurs, the only way to minimize it quickly is by providing for the safe exhaust of hot gases. Existing methods have not adequately solved the problem. Accordingly, there is a need for an arc resistant cabinet having means for directing high temperature, expanding gases away from the cabinet whereby said means do not suffer from delay in opening and thus release of hot gases. There is also a need for providing a plenum having means to direct the hot exhaust gases from the arcing section away from the other non-arcing sections to avoid secondary fault. And a further need is felt to provide for a plenum assembly where the exhaust gases are thrown out of the area enclosing said arc resistant cabinet efficiently and safely and without causing any leakage there within.